4.5: Measuring Earthquakes
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Machines to measure earthquakes are seismometers. But, confusion exists between what a seismometer and a seismograph are because, before computers, there needed to be both a sensor to measure the seismic waves and a recording device. The sensor is the seismometer and it was usually buried, and out-of-sight, while the recording device, the seismograph was visible. It was a drum with paper and either a stylus or a pen that marked the paper as the drum turned, recording the seismic waves (Figure \(\PageIndex{1}\)). The data record is a seismogram, the sensor measuring the seismic waves is the seismometer, and the recording device is the seismograph. Because people saw the seismograph, and seldom actually saw the seismometer, it became common to refer to the seismograph as if it were actually the seismometer.
Fast forward to today, and the situation has changed dramatically. Seismographs are seldom used, the signal measured by the seismometer is sent directly to a computer and stored digitally. Also, seismometers have greatly improved. Older seismometers could measure one direction of motion, so to record the movement of the Earth in 3-dimensions required 3 seismometers, each positioned along a different axis - one vertical, one horizontal and oriented north-south, and a second one horizontal, but oriented east-west (Figure \(\PageIndex{2}\)). Now all three seismometers are packaged into one container.
Pendulum seismometers were some of the earliest designs and are based on the inertia of the suspended mass. They were generally replaced by electromagnetic seismometers by the mid 1900s. These still use a mass, but instead of being suspended by a wire, the mass is attached to a spring. The motion of the mass is measured by an electromagnet. A practical difference is the seismometer is now small enough to be portable whereas pendulum seismometers are large, permanent installations. Portable seismometers are valuable for detecting earthquakes in new locations or when previously quiet areas, such as volcanoes preparing to erupt, become seismically active. They are also useful for investigating the geology of relatively unexplored regions to detect local earthquake activity too small to be recorded globally.
A critical difference among electromagnetic seismometers is they can be designed to measure different frequencies. Local earthquakes have higher frequency data than distant earthquakes. These teleseismic events have mostly lower frequency data because the higher frequency information attenuates or weakens with distance. Therefore, measuring an earthquake that occurred at a great distance and not being able to record the high frequency information means that the energy from the earthquake will be underestimated, as will its magnitude.
Seismometers therefore are often referred to as short period for local earthquakes, long period for measuring distant earthquakes, or broadband when they record a broad band of frequencies and can be used to study either local or distant earthquakes. They can also be deployed either on land (usually in the ground; Figure \(\PageIndex{3}\)) or on the sea bottom (Figure \(\PageIndex{4}\)).
Other differences between modern and older seismology is that paper seismographs are for the most part no longer used. Data are now digital, not analog. The data are either recorded on a digital recording device which is periodically retrieved, or sent by telemetry to an offsite computer. This has changed the processing and interpretation of seismic data dramatically. It’s much easier to compare data on different earthquakes digitally. Also analog/paper records (or their digital equivalent, webicorder records) can have the problem of the signals from multiple earthquakes overlapping each other and making them hard to interpret. A digital signal however doesn’t have to be viewed like a paper record or a webicorder display. It can be viewed over time, removing the overlap problem. This problem is illustrated in Video \(\PageIndex{1}\) on webicorder displays.
The following video explains how to read seismograms.
Today most analog records are more for public display rather than earthquake research. Data are recorded digitally, transmitted back to a university or research center for use on a computer. They may be displayed as webicorder records, but are usually interpreted digitally. A video of a museum display in Canberra, Australia, summarizes how seismic data are collected and used.
A visit to a museum in Canberra, Australia reviews how earthquake data are collected and how they are used.
California Seismic Networks
If the information from one seismometer is useful, imagine how much more useful the information from multiple seismometers might be. One example that has already been discussed is finding the epicenter of an earthquake, which takes a minimum of three stations. The data recorded by seismic networks present opportunities for finding out much more about Earth than data from a single station (Figure \(\PageIndex{5}\)).
Networks for data collection are important. The more stations in the network, the more data collected and the better the understanding of the earthquake and the surrounding geology. The way the network is designed is important. If the seismic stations in the network are widely dispersed, information can be gathered on smaller earthquakes everywhere within the network. The closer the stations, the smaller the earthquakes that can be recorded on multiple seismometers. The magnitude of completeness (Mc), or the smallest magnitude earthquake that can be confidently recorded by a network, is determined by the station density. If an “earthquake” is recorded on only a single seismometer, it may be wind, or traffic or other noise, or it’s too small to be substantiated.
An important advantage of having a network of seismic stations is that it makes it possible to develop early warning systems that inform people that an earthquake has started.
One of the results of having strong seismic networks has been the creation of an earthquake early warning system. It’s called ShakeAlert and extends along the west coast from British Columbia in Canada to Mexico. ShakeAlert does not predict earthquakes. Data being transmitted from a seismic station to a processing center are automatically analyzed and screened to see if an earthquake has occurred. If a possible earthquake is detected, the data from nearby stations are queried to see if they are also recording an earthquake. If that appears so, the automated system quickly calculates a first approximation for the location and size of the quake (see Locating an Earthquake). The U.S. Geological Survey then sends the information out to organizations and agencies that have it distributed as an alert to mobile phones in the areas where shaking is most likely to occur.
The following video offers a more detailed look at the West Coast’s earthquake warning system.
It works because computers can do all of this analysis very quickly, and electromagnetic waves, such as radio waves, move at the speed of light (approximately 300,000 km/s), whereas the seismic waves move much slower (5 to 8 km/s, depending upon the rock). Once an earthquake is detected, it’s possible to warn people that an earthquake is happening, and the farther away, the more warning time. It does however take time for an automated system to make its predictions, so the system is not perfect and people closest to the earthquake may experience shaking before they get a warning. The farther the location is from the earthquake, the more advanced the warning. For more information about the ShakeAlert system see Earthquake History and Hazards. Also, the ShakeAlert Website has information on how to sign up for and use the ShakeAlert system.
California has three main seismic management networks - CGS, NCSN, and SCSN. They are organized under the Advanced National Seismic System (ANSS) operated by the U.S. Geological Survey. There are more than 15 seismic networks within the state that share data and report to the three main management networks.
The California Geological Survey (CGS) operates a network of over 900 accelerometers. Accelerometers are a type of seismometer that collects data on the acceleration of the ground motion. Most seismometers collect information on the velocity of the ground motion. Acceleration information is especially important to engineers designing structures because it measures how fast changes in velocity occur. If a structure cannot respond well to standing still, then suddenly being in motion, it is likely to fail (Figure \(\PageIndex{6}\)).
Established in 1967, the Northern California Seismic Network (NCSN) is managed jointly by the U.S. Geological Survey and the University of California, Berkeley and operates 580 stations in northern and central California, along with incorporating 159 stations from the Southern California Seismic Network, and networks maintained by California state agencies, the University of Nevada, Reno, and public utility companies. It also includes additional seismic stations in the Pacific Northwest and Nevada because of its affiliation with the U.S. Geological Survey’s California Volcano Observatory. A more detailed description of the network, including a current map of station locations can be found at the USGS Northern California Seismic Network.
Approximately 60% of the network is digital and most of the stations are short-period, vertical-component seismometers. The magnitude of completeness (Mc) varies from Mc 1.0 in the central Coast Ranges to Mc 2.6 in the Klamath Mountains. Data from the network are distributed by the Northern California Earthquake Data Center and by EarthScope Data Center (IRISMC).
The Southern California Seismic Network (SCSN) is the oldest seismic network in California. It was established in 1926, and it has been jointly managed by the California Institute of Technology (Caltech) and the U.S. Geological Survey since 1970. As of 2008, it has more than 400 stations and extends south from a line between San Luis Osbipo to Big Pine, California then continues south to Mexico. It has an approximate Mc 1.8. A detailed description of the network, including a current map of station locations can be found at Southern California Seismic Network. Data from the network are distributed by the Southern California Earthquake Data Center and by EarthScope Data Center (IRISMC).
Earthquake Intensity and ShakeMaps
Intensity is the scale that has been historically used to measure earthquakes. It is based on the damage done by earthquakes and intensity values are reported using Roman numerals (Table \(\PageIndex{1}\)). People experienced earthquakes long before instrumentation was invented to measure earthquakes, and therefore what could be measured was the effect earthquakes had on structures and the landscape. There are, however, problems with any intensity scale. Building materials and construction methods vary around the world, therefore intensity scales are modified and not the same world-wide. The intensity scale used in the United States is the Modified Mercalli Scale (Table \(\PageIndex{1}\)), which is the 1931 modification of a scale originally developed by the Italian volcanologist Giuseppe Mercalli.
| Intensity | Shaking | Description/damage |
|---|---|---|
| I | Not felt | Not felt except by a very few under especially favorable conditions. |
| II | Weak | Felt only by a few persons at rest, especially on upper floors of buildings. |
| III | Weak |
Felt quite noticeably by persons indoors, especially on upper floors of buildings. Many people do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibrations similar to the passing of a truck. Duration estimated. |
| IV | Light |
Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motor cars rocked noticeably. |
| V | Moderate | Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects overturned. Pendulum clocks may stop. |
| VI | Strong | Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight. |
| VII | Very strong | Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken. |
| VIII | Severe | Damage slight in specially designed structures; considerable damage in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned. |
| IX | Violent | Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations. |
| X | Extreme | Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Rails bent. |
Another concern is that an intensity scale cannot be used to compare one earthquake with another earthquake. For example, if a building is damaged by an earthquake, and then a second earthquake occurs a few days later and the building now collapses, does this mean the second earthquake was larger? Or does it mean that the first earthquake damaged the building such that it could collapse if a smaller earthquake occurred? Also the damage caused by earthquakes is dependent upon the distance to the earthquake. If damage caused by two earthquakes is different, might it be because one earthquake was closer rather than bigger? Questions such as these led seismologists to find another way to measure earthquakes, such that one earthquake could be compared with another. Even though intensity is not a good way to compare one earthquake with another, it’s a great way to compare the earthquake with itself. Sometimes, a video can do a better job of describing motion than words.
In the following video, light bulbs are used to explain the concept of intensity and how intensity is used to describe earthquakes.
Intensity depends on multiple factors:
- how big the earthquake was
- how far away the earthquake was
- the local geology
- duration of shaking
- construction of buildings
The bigger the earthquake the larger the energy release. The farther from the earthquake, the more earthquake energy will attenuate or weaken. The local geology depends upon many factors. If the shaking lasts longer, the potential for damage is greater, and usually more damage occurs (Figure \(\PageIndex{7}\)). If a building or structure is not constructed to withstand motion, it is more likely to experience damage (see California's Earthquake History and Hazards).
Pretend an earthquake occurs. You and a friend are in different locations, but are equally distant from the earthquake. One of you is on campus, in class and in a well constructed building built on strong igneous rock. The other one has no classes that day and is at the beach. Who will experience the most shaking and probably see the most building damage?
Interestingly enough, because of the local geology, the person at the beach will probably experience more shaking and is more likely to see more damage to structures. The strong rock will most likely move as a unit and that minimizes the shaking and the shearing that can happen to structures. The loose beach sediments have no internal connections to give them strength and can move every which way, which will also cause seismic waves to slow down. Therefore, when the seismic waves encounter a material such as beach sands, the amplitude of the waves increases which will magnify the shaking. Often in areas of loose, wet sediment or weak, water-saturated sedimentary rock, liquefaction can occur (see Liquefaction and the Role of Soil Type). Many people have the misconception that the loose beach sands will act more like “packing peanuts” and somehow protect structures. Packing peanuts protect the contents of packages because they are packed in a box very tightly and have no room to move. Sand on the beach has no such constraints.
Intensity is usually mapped as an isoseismal map or ShakeMap. Sometimes the information is displayed by drawing isoseismal lines, or contour lines of equal intensity, as in the Interactive Isoseismal Map for the 1994 Northridge Earthquake or as colors on a gradational scale (Figure \(\PageIndex{8}\)) with warm colors (reds, in this example) for the areas of highest intensity to cool colors (blues, in this example) for areas of lowest intensity. The pattern of gradational color changes or of contour lines draws attention to the areas of damage. This information can then be compared with other information such as geology maps or maps of local building zones to help determine why these areas experienced damage and to determine if measures such as new building codes or zoning restrictions could help prevent future damage.
For earthquakes in the United States or large earthquakes globally the U.S. The U.S. Geological Survey’s Earthquake Hazards Program maintains a website, Latest Earthquakes, where data on historical and recent earthquakes are available, including ShakeMaps, and also includes an opportunity for people to contribute to earthquake intensity data by reporting their experience during an earthquake - Did You Feel It?
Earthquake Magnitude
Earthquake magnitude was designed as a way of measuring earthquake strength using properties of the earthquake as measured on seismometers, rather than on damage done by the earthquake. While there are multiple scales used to measure magnitude, most news media refer to any magnitude scale as the Richter Scale because Charles Richter’s original scale was widely used and accepted for decades.
In 1935, American seismologist Charles Richter developed the first scale to measure what is now referred to as local magnitude or ML. This measurement is based upon the amplitude of seismic waves as recorded on seismometers with corrections applied for distance to the origin of the earthquake. A problem eventually became apparent with this scale. Very large earthquakes occur when there is a long length of fault that ruptures. This energy is long period, and long period data are not part of the calculations used in determining a Richter magnitude. This means the original Richter magnitude scale underestimates the size of large earthquakes and the larger the earthquake, the larger the underestimation.
The moment magnitude scale or MW was developed in 1979 and measures earthquakes based upon what is called the seismic moment in addition to the ground motion as measured on seismograms used to calculate the Richter or local magnitude ML. The seismic moment is also incorporating information from the geometry and motion of the fault and properties of the rock.
Both scales are logarithmic with respect to ground motion, and every whole number increase represents an increase of 10 times greater ground motion than the previous number. For smaller earthquakes, magnitude 5 or less, the magnitude is approximately the same on either scale. However as earthquakes get larger than a magnitude 5, the moment magnitude gets progressively larger much more quickly than the Richter magnitude does. Therefore, the larger the earthquake, the greater the difference between the two magnitude scales and the greater the underestimation of the magnitude when using the Richter scale. Because of this, the moment magnitude scale has become the standard for reporting large earthquakes and has replaced the Richter and other magnitude scales.
The following video provides a more detailed explanation of how earthquake magnitudes vary and why some are better than others for trying to estimate the energy released during an earthquake.
The moment magnitude scale was designed to better understand the energy released during earthquakes. One unit of magnitude is 10 times more ground motion and this increase in ground motion is from 32 times more energy being released. Therefore a magnitude 6 earthquake requires 32 times more energy than a magnitude 5 earthquake. An example of this can be illustrated using dry spaghetti noodles (Video .
A detailed example of how moment-magnitudes are calculated and why it takes 32 times the energy release to create 10 times the movement when an earthquake occurs.
When the Richter scale was being developed, Richter was using the best available equipment available. However, seismometers have greatly improved since the 1920s and 1930s. Therefore a “0” on the Richter scale back then meant no motion measured. Richter understood that there could be earthquakes that small, but they could not be reliably measured on the equipment used then. Today seismometers are much more sensitive and magnitude "0” earthquakes can easily be measured, in fact, magnitude -1 and -2 can be measured and detection levels are approaching -3 earthquakes. Humans are lousy earthquake detectors, the smallest earthquakes most humans can experience are magnitude 2. Therefore, today’s seismometers can measure earthquakes 10,000 times smaller than a human can experience.
Earthquake Risk
If you can measure an earthquake, why can’t you predict them? It’s complicated. Earthquakes have been rarely predicted, but not consistently. They have also been mis-predicted. The problem is understanding just when a bit of fault will break, where this will be, and then how will the rupture propagate? The challenge is illustrated by two earthquakes that occurred in China during the 1970s. The M7.3 Haicheng earthquake of February 4, 1975 was successfully predicted; the MW7.6 Tangshan earthquake of July 28, 1976 was not predicted and approximately 240,000 people died. Two earthquakes, less than two years apart, in the same country, so the protocols and technology are comparable. Why wasn’t the second one predicted?
The easiest way to approach that question is to look at why the first one was. It had a foreshock sequence that was recognized as such, the second earthquake did not have a foreshock sequence. If seismicity were the same before all earthquakes, prediction would be much easier, but it's not. Earthquakes vary in tectonic setting and geology and no one-size approach works.
In the 1970s, the fact that a major earthquake had been predicted made more scientific organizations acknowledge that it was possible under the right conditions; the fact that the next major earthquake was not predicted, made these same agencies more cautious and steered earthquake awareness in a different direction - increasing the emphasis on earthquake preparedness.
This video provides more detailed explanation of how earthquake magnitudes vary and why some are better than others for trying to estimate the energy released during an earthquake.
Video \(\PageIndex{6}\) illustrates some of the problems with earthquake prediction, a major one being that while many things are known about what can happen before an earthquake, why do they happen sometimes and not happen other times? Today, many seismologists believe that predicting earthquakes will never be entirely successful. Therefore, while research continues to try and learn more, the questions become - What is known? And what can be done to make the best use of that information?
What is known? Where earthquakes occur, how large an earthquake is possible, and what amount of ground shaking will occur, but not when an earthquake will happen or how large it will be (Figure \(\PageIndex{9}\)). Therefore, the approach that is being taken by both the state and the federal government is to try and prepare for earthquakes in areas where there is a risk of major earthquakes and to continue to try and learn more about earthquakes in the hope that one day earthquake prediction can become a reality. For more about the hazards presented by earthquakes and how to prepare for an earthquake see California's Earthquake History and Hazards.
References
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- Wald, D. J., Quitoriano, V., Worden, C. B., Hopper, M., & Dewey, J. W. (2012). USGS “Did You Feel It?” Internet-based Macroseismic Intensity Maps. In R. Bossu & P. S. Earle (Eds.), On the Use of the Internet to collect earthquake information. Annals of Geophysics 54(6) 688-707. DOI:https://doi.org/10.4401/ag-5354